1. Field of the Invention
This invention relates to an image processing method and apparatus and, more particularly, to an image processing method and apparatus through which the resolution of binary image data can be improved. By way of example, the invention relates to an image processing apparatus applied to image processing in an image output unit such as a digital printer or digital facsimile machine.
2. Description of the Prior Art
An image output unit such as a digital printer or digital facsimile machine most generally is a binary output (i.e., a "black" or "white" output) device for reasons of economy and safety. In a binary output unit of this kind, pseudo-half-tone processing, described below, is necessary in order to output a gray-level image.
The most typical technique employed in the art is the dither method. In accordance with the dither method, an m.times.n (where m, n are natural numbers) dither matrix is prepared, a binary measurement is performed by comparing inputted multivalue data with the threshold values of corresponding matrix elements, and an m.times.n binarized block is formed, whereby a half-tone image is reproduced in pseudo-fashion.
With the dither method, however, the number of gray levels that can be expressed is limited to (m.times.n+1). This is undesirable in terms of resolution.
By contrast, an error diffusion method, which was proposed in 1975 by Floyd and Steinberg in the technical paper "An Adaptive Algorithm for Special Grayscale", which appeared in the 1975 SID International Symposium Digest of Technical Papers, surpasses the dither method in terms of both resolution and gray level. This technique has recently become the focus of much attention.
In the error diffusion method, binarization is performed at a fixed threshold value, a corrective density is obtained by adding, to the density of a pixel of interest, a diffusion error from a pixel in back of the pixel of interest (i.e., from a pixel processed before the pixel of interest), the difference is calculated between this corrective density and a binarized density (e.g., "white"=0, "black"=255 when density is expressed using eight bits) resulting from the aforementioned binarization, and this difference is caused to diffuse as a new error to a pixel in front of the pixel of interest (i.e., to a pixel which has not yet been processed).
However, in a reproduced image obtained by pseudo-half-tone processing such as the above-described error diffusion method, the graininess of low-density areas (highlighted portions), to which the human eye is most sensitive, becomes unsightly, and this is a factor that impedes further improvement in picture quality.
Though graininess diminishes when the resolution of the output device is improved, the improvement in resolution is accompanied by the need for an increase in the size the page memory of a page printer or the like having such a memory, which is for storing one page. The result is higher cost.
By way of example, if resolution in the main scanning direction is doubled, twice the amount of page memory will be required. If resolution is doubled in both the main and subordinate scanning directions, four times the amount of page memory will be required.